The solar cells currently available in the market, based on which kind of material is used, include crystalline silicon, a compound and organic types of solar cells. In comparison with the others, the manufacture of organic solar cells is relatively simple and the manufacturing cost is inexpensive. Organic solar cells can be additionally categorized into dye-sensitized solar cells and organic thin film solar cells according to their different device structures. The organic material generally used in organic thin film solar cells has a light absorption coefficient of about 105 mol−1 cm−1. This excellent property of light absorption means that organic thin film solar cells can deal with the sunlight much more efficiently. However, the organic solar cells presently developed are poor in their power conversion efficiency (PCE) and have short cell lives, and so the commercial value of utilizing the organic solar cells is reduced.
Accordingly, to improve the power conversion efficiency of polymer solar cells (PSCs) and to reduce the manufacturing costs for commercial purposes, solar cells should possess at least the following properties: (1) a high absorption capacity and a broad absorption band compared to sunlight; (2) high charge mobility; (3) a low energy level of the highest occupied molecular orbital (EHOMO); and (4) good solubility. A polymer solar cell having good properties (1)-(3) is able to achieve the performance of a high short-circuit current, a high open-circuit voltage and high power conversion efficiency, and property (4) enables the possibility to produce solar cells at a lower cost by using a wet process assisted by a solvent and to manufacture solar cell products on a substrate having a large area.
The solubility in a solvent and the solid structure of a conjugated polymer compound can be improved by (1) modifying the main chain structure to improve the absorption capability to the spectrum of sunlight, improve the carrier mobility and modulate the energy level of a molecular orbital; and (2) modifying the alkyl group on a side chain of the main chain. The conjugated polymer used in the past, such as poly(3-hexylthiophene), has poor collection capacity for photons of sunlight in the red light region and near infrared light region, for which the wavelength in sunlight is between 600 nm and 900 nm, so the absorption efficiency is hard to improve. A design to use a polymer having a low bandgap can shift the absorption band of the material to the wavelength in the region from 600 nm to 900 nm to overcome the problem mentioned above, however, some new problems have arisen. The new problems include poor absorption of wavelengths between 400 nm and 600 nm, meaning that the light in short and medium wavelengths between 400 nm and 600 nm, which has a higher energy, cannot be effectively utilized. In addition, light in longer wavelength regions, higher than 900 nm, cannot be effectively utilized either.
A porphyrin compound can effectively absorb sunlight in the short wavelength region, making it an ideal functional unit to improve the absorption of a conjugated polymer for sunlight. It can be observed in the prior art that the introduction of a porphyrin compound to a polymer material having a lower bandgap or to a traditional conjugated polymer will cause deterioration to the performance of the solar cell. The reason is that the porphyrin compound is a bulky cyclic molecule, and the bulky molecule destroys the conjugation structure of the main chain in the conjugated polymer, so the absorption in the red light region, with a wavelength between 600 nm and 900 nm, is reduced, and the current collected by the solar cell is relatively low. Furthermore, the introduction of the porphyrin compound will vary the solid stacking of the conjugated polymer, influence the morphology of the active layer in the solar cell, and result in a decrease in the fill factor (FF) of the solar cell. Due to the reasons above, both the power conversion efficiency and the stability of the polymer solar cell containing the porphyrin compound are quite low. Accordingly, a polymer material with the absorption in the ranges close to the full spectrum of sunlight with high efficiency and a light-sensitized material containing the porphyrin compound have not yet been applicable to polymer solar cells.
In order to overcome the drawbacks in the prior art, a conjugated polymer compound is disclosed. The particular design in the present invention not only solves the problems described above, but is also easy to implement. Thus, the present invention has utility for the industry.